Alumina chemically combined with 28.2% by weight silica in the general formula 3Al.sub.2 O.sub.3.2SiO.sub.2 is a composition known as mullite. This composition exhibits a characteristic X-ray diffraction pattern, has many of the known valuable properties of alumina and, in addition, exhibits other valuable physical and chemical properties. When mullite is formed as whiskers, the unusual strength associated with single crystals is obtained.
Various methods have been suggested in the prior art for the production of mullite in whisker or fibrous form. For example, U.S. Pat. No. 3,104,943 discloses methods whereby mullite fibers of less than 5 microns in cross-sectional diameter and an axial ratio of at least 100 to 1 are produced. The process is a vapor-phase reaction at a temperature of from 800.degree. C. to 1,200.degree. C. in which silicon dioxide, aluminum, and aluminum combined with a source of sulfur are reacted in an atmosphere containing at least 1% hydrogen.
U.S. Pat. No. 3,023,115 discloses a process for the production of a composition of matter, in the form of discrete fibers consisting essentially of between 95% and 99% by weight alumina, and 1-5% by weight silica. The process of this invention comprises reacting a gaseous suboxide of aluminum with silicon monoxide vapors in a hydrogen atmosphere at a temperature of from about 1,370.degree. C. to about 1,500.degree. C.
U.S. Pat. No. 3,321,271 discloses a process for production of aluminum silicate whiskers in a vapor state reaction carried out at a temperature of from 1,000.degree. C. to 1,400.degree. C., while U.S. Patent 3,607,025 discloses a process by which an alkali metal halide and aluminum chloride are contacted with silica in the presence of an oxidizing gas at a temperature of from 1,000.degree. C. to 1,350.degree. C. to form silica deficient mullite fibers.
The formation of mullite fibers from the reaction of aluminum trifluoride and silicon dioxide was reported by D.A. Haught of the U.S. Navy Surface Weapons Research Center at a non-public meeting sponsored by the USACA and NASA in Cocoa Beach, Fla. in January 1988. Subsequently, on Mar. 20, 1990, U.S. Pat. No. 4,910,172 issued to Talmy and Haught. The Talmy et al patent is directed to a method of preparing mullite whiskers from AlF3, Al.sub.2 O.sub.3 and fused silicon powders, wherein the reactants pass through a topaz crystal form, which crystals are heated in an SiF.sub.4 atmosphere to form the mullite whiskers. Talmy et al are also the listed inventors on PCT Published Application WO 90/01471, which claims priority based on U.S. application 229,517, filed Aug. 5, 1988. This PCT Publication is directed to processes for producing rigid mullite whisker felt for pre-forms and thermal insulators. Work involving the manufacture of crystals according to this route has also been conducted by Suvorov and others at the Leningrad Technical Institute. Prior to the present invention, however, this reaction of aluminum fluoride with silica held together with a binder, has not been employed to form complex-shaped articles, such as catalyst supports which should be fluoride-free.